In recent years, a plastic film used for packaging food is strongly required to have a vapor and oxygen-proof characteristic (barrier property). In order to provide a high barrier property to a sheet material such as a plastic film, there is a need for coating the sheet material with a transparent film of SiOx, Al2O3, or the like. Conventionally, as a coating technique of a SiOx film, there are physical vapor deposition methods (PVD methods) such as a vacuum vapor deposition method and a sputtering method. However, in recent years, a plasma CVD method superior in terms of deposition speed and deposition of a high barrier coating in comparison to the above techniques is used.
As a plasma CVD apparatus used in this plasma CVD method, for example there are known apparatuses shown in Patent Documents 1 to 3.
A plasma CVD apparatus described in Patent Document 1 utilizes Penning discharge, and includes a vacuum chamber and a pair of deposition rollers provided in an interior of this vacuum chamber. These deposition rollers convey a substrate in a state that the substrate is wound between the deposition rollers. One pole of a high-frequency AC power supply is connected to these deposition rollers in such a manner that the deposition rollers have the same polarity. Meanwhile, an anti-pole of an annular electrode is disposed separately from the deposition rollers at a position which is away from a center of a space between the deposition rollers (hereinafter, referred to as the inter-roll space) by a substantially equal distance to each other. The other pole of the high-frequency power supply is connected to this anti-pole.
In this plasma CVD apparatus, by supplying high-frequency power of tens of or hundreds of kHz between the deposition rollers and the anti-pole, a magnetic field is generated so as to range between the deposition rollers, and discharge (Penning discharge) is generated in the inter-roll space. This discharge ionizes a source gas in the vacuum chamber and generates plasma so as to realize plasma CVD. That is, a coating is deposited on a surface of the substrate wound between the deposition rollers.
However, in this apparatus of Patent Document 1, there is a possibility of a trouble caused by deposition on the anti-pole. That is, in this apparatus, the annular electrode is used as the anti-pole in addition to the deposition rollers, and the power is supplied to this anti-pole from the high-frequency power supply. Thus, there is a fear that plasma is also generated and a coating is deposited on a surface of the anti-pole. There is a possibility that the coating deposited on the anti-pole in such a way is peeled off and mixed into the original CVD coating as flakes, so that quality of the CVD coating is lowered.
Patent Document 2 discloses a plasma CVD apparatus which is different from the apparatus described in Patent Document 1 in which the anti-pole is provided in addition to the deposition rollers and the power supply is connected between these, the plasma CVD apparatus including first and second deposition rollers provided in line in the horizontal direction, capable of generating plasma with one of the deposition rollers serving as an acting pole and the other deposition roller serving as an anti-pole. Specifically, an anode and a cathode of a plasma power supply are respectively connected to the first and second deposition rollers, and thereby, both the deposition rollers are alternately used as the acting pole and the anti-pole. A substrate is wound around surfaces of the deposition rollers. Thus, even when a CVD coating is deposited on the substrate, due to the deposition rollers serving as the only anti-pole, generation of flakes due to the deposition is suppressed.
Particularly, in this plasma CVD apparatus described in Patent Document 2, a space between both the deposition rollers is physically isolated from other spaces so as to form a discharge chamber, and plasma is generated and a coating is deposited mainly inside this discharge chamber. Thus, the generation of the flakes on the outside of the discharge chamber is prevented. Specifically, partition walls surrounding the discharge chamber are opened toward the sides of the deposition rollers. The openings of the discharge chamber are closed by the deposition rollers, and a vacuum degree in the discharge chamber is maintained at a pressure suitable for generation of plasma by supplying a source gas, so that the deposition is performed. Thereby, discharge is performed only in the discharge chamber. Thus, the generation of the flakes on the outside of the discharge chamber is prevented.
However, in the deposition apparatus described in Patent Document 2, although the flakes are not generated on the outside of the discharge chamber, the coating is easily attached to wall surfaces surrounding the discharge chamber, and there is a fear that the flakes generated on the wall surfaces are mixed into the CVD coating, so that quality of the coating is lowered. Further, in order to perform the deposition in this apparatus, there is a need for supplying the source gas into the discharge chamber so as to lower the vacuum degree in the discharge chamber. Therefore, there is a need for maintaining a gap between the partition walls surrounding the discharge chamber and the deposition rollers to be small so as to retain airtightness of the discharge chamber. However, when the coating is accumulated in the vicinity of this gap and size of the gap is changed, the airtightness of the discharge chamber is not easily retained and stability of the deposition is deteriorated. Thus, there is a fear that the quality of the CVD coating is lowered.
Patent Document 3 discloses an apparatus including a pair of hollow deposition rollers and magnetic field generating means provided inside the deposition rollers for generating a magnetic field, wherein plasma is generated only in a magnetic field region formed by the magnetic field generating means. Each of the magnetic field generating means has an N-pole magnet and an S-pole magnet. These magnets are spaced from each other so as to protrude toward a space between the deposition rollers, and form a magnetic field having a magnetic line coming from one magnetic pole in the deposition roller to an exterior of the deposition roller and returning again to the other magnetic pole in the deposition roller. Plasma is generated only in a region along the magnetic field.
With this deposition apparatus described in Patent Document 3, the region where plasma is generated can be limited to part of surfaces of the deposition rollers where the magnetic field is generated. However, the plasma reaches to a supplying portion of a source gas and is accumulated in the vicinity of the supplying portion so as to be flakes, and the flakes are dropped and mixed into a CVD coating. Thus, there is a possibility that quality of the coating is adversely influenced.